2D Sediment Transport (2022)

The class kicks off with six mini-case studies, including laboratory and prototype models that highlight the capabilities of the 2D sediment capabilities in HEC-RAS.

A good 2D sediment model starts with an excellent 2D hydraulic model.  Sediment results are more sensitive to good hydraulic and mesh practices than hydraulic water surface or floodplain analysis.  This session introduces some of the 2D mesh and hydraulic best practices that precede sediment modeling, including mesh alignment and  break lines.  We will also cover model warmup and turbulence approaches, as well as some of the new diagnostic tools to identify problematic cells and model instabilities.

Sediment results are more sensitive to good hydraulic and mesh development than hydraulic water surface or floodplain analysis.  This session introduces some of the 2D mesh best practices - including channel alignment and structure definition - that set up a good sediment, as well as some of the new diagnostic tools to identify mesh issues.

Gibson

The 2D model in HEC-RAS uses a disequilibrium, advection-diffusion approach, with is fundamentally different than the 1D algorithms.  This presentation will introduce the differences between equilibrium and disequilibrium approaches to sediment modeling.  It will also introduce the governing equations, the advection-diffusion approach, how the erosion and deposition rate terms work, and the 2d sediment transport equations (van-Rijn, Wu, and Soulsby-van Rijn).

Sanchez

This presentation will introduce the basic sediment inputs in HEC-RAS required to run a model: mainly bed gradation and sediment boundary conditions.  We will describe how to define bed gradations spatially in RAS Mapper with a Bed Gradation Layer and how to connect it to sediment data in the Sediment Editor.  This session will also introduce the 2D sediment boundary conditions. Then we will introduce the sediment results tools in RAS Mapper, including how to add and access sediment results, and some of the most helpful results to evaluate different models.

Gibson

Most of the runtime in 2D sediment models is associated with solving the sediment and bed mixing equations for each grain size.  However, recent versions of HEC-RAS include a 2D sediment “Capacity Only” approach, that compute fixed-bed, sediment capacities and can route sediment concentrations without erosion or deposition. These tools can provide sediment process insights at runtimes comparable to the hydraulic model.   This session introduces 2D, fixed bed, sediment modeling options and discusses their potential for screening alternatives and reducing the number of mobile bed simulations required.

The 2D sediment model has four “warm-up” phases.  The standard, 2D, hydraulic warm up is even more critical for sediment models.  But in addition to that phase, the sediment model can initialize the concentration, bed gradation, and/or the bed elevation.  Initializing the bed so the bed gradation, hydraulics, sediment flux, and transport function are in quasi-equilibrium at the beginning of a simulation will help the modeler to isolate the impact of management alternatives or morphological events from numerical artifacts of initial conditions.  This session will also discuss how to limit the computational burden of this warmup phase with a hotstart file. 

Gibson

Subgrid bathymetry is the unique feature of the HEC-RAS 2D approach.  It is difficult to model 2D hydraulics well without understanding how the subgrid approach works.  But 2D sediment can also utilize subgrid, computing bed change and bed mixing for multiple regions within each cell.  This session introduces the sediment subgrid model in HEC-RAS, describes how to opt in or out of sediment subgrid, discusses the computational costs, and demonstrates how to plot and interpret subgrid sediment results.

This presentation will describe the Advection-Diffusion parameters and help students understand when to use them.  The presentation will describe the somewhat unique parameterization of the advection term in RAS and its advantages.  Then it will focus on the parameters, including the Load Correction Factors (Total, Bed, Suspended) that and the Diffusion Coefficient options.

Sanchez

This presentation describes the features under the “Erosion” tab of 2D Sediment parameters.  This will primarily focus on choosing an Adaptation Length but will cover the Total Length (most common) approach, and partitioning by bed and suspended components.

Sanchez

This presentation will describe other 2D options and parameters including outer loop convergence, base bed slope parameters, avalanching, splash erosion, and other options.

HEC-RAS 2D Sediment has the classic cohesive options in HEC-RAS and HEC6/6T.  But it also has several additional options, including flocculation and consolidation.  This presentation introduces the cohesive options available in 2D Sediment.

This presentation will explore several common problems 2D sediment modelers encounter and best practices to resolve or work-around them.